Fix documentation examples

[lexnv]

This PR ensures that the cargo test --doc is passing.

While at it, added a rule for the CI to run this command.

[jsdw]

I love the fixed doc examples! My vote on this PR would be to remove the rusty example bits from the markdown file and merge the rest!

[lexnv]

Have removed the docs folder in favor of the docs.rs documentation. Thedocs/subxt.md is now part of the src/lib.rs main documentation, that's a bit better in terms of readability of the examples 😄

[jsdw]

A couple of wee nits but looks great, nice one!